The invention disclosed herein pertains to a method and apparatus for detecting a person or object in an aisle between mobile storage units and for automatically stopping movement of the unit before a collision with a person or object occurs.
Mobile storage units are well-known and are described in many patents including U.S. Pat. No. 4,557,534, which is owned by the Assignee of this application. A mobile storage unit comprises a carriage that supports shelving and is driven bidirectionally on tracks by a motor mounted on the carriage. Such units are used in libraries, archival record rooms, storage rooms, and offices for storing records, documents and books and mobile units are frequently used in warehouses and industrial plants for storing parts and finished products. One of several meritorious features of mobile units is that they conserve floor space by reason of requiring only one aisle to be open between any two units at one time, since all other units on the same tracks or rails are driven into abutting relation with no aisle space between them.
If an object or person is in an open aisle at a time when another person attempts to open a different aisle the movement of all units will be arrested by the primary safety protective system herein proposed. A conventional way of achieving this safety objective has been to mount a swingable sweep bar near the bottom of each mobile unit and extending over the width of the unit. If the sweep bar is pressed by a moving unit pushing an object against it, or if it is pressed by the foot of a person in an aisle who observes a mobile unit moving toward him or her, the sweep bar activates switches that disable the carriage drive motors.
Supplanting sweep bars with photoelectric systems has advantages as explained in U.S. Pat. No. 5,121,975, which is owned by the Assignee of this application. In the patented system, analog electric circuitry is used. A light beam transmitter is mounted on the aisle side of a mobile unit near one end of the unit, and a photosensitive receiver is mounted remotely near the opposite end of the unit. The beam of light transmitted by the transmitter is focused on the photosensitive receiver. If the light beam is interrupted by a person or an object, electronic circuitry responds by deenergizing the carriage drive motor.
Several problems are encountered in photoelectric based safety sweep systems. One problem is that transmitters from different manufacturers and even from the same manufacturer have variances in their light beam output intensities which must be accounted for in the electronic circuitry. The sensitivity of the receivers also varies substantially. Even transmitters and receivers in a batch obtained under the same model number from the same manufacturer have variations in their characteristics.
Another problem arises from the fact that a photodetector system that works well when it is installed on a carriage having a certain length may not work well on a unit at another installation where the units have a greater length. The problem is inherent in the fact that the beam intensity at the receiver varies in some proportion to the distance between the transmitter and receiver. For example, some mobile storage units may have widths of three or fewer meters while others may be as wide as thirty meters. It is problematical to design a protective system whose sensitivity is so great that it is affected in the same manner by a normal beam intensity and a normal beam intensity plus interfering stray or ambient radiation. As a practical matter, it is desirable for a manufacturer of photoelectric protective systems to only be required to design and have available a few models that are suitable for mobile units having large differences in their widths. Otherwise, it would be necessary for the manufacturer to inventory a variety of protective devices having different electronic circuitry and different transmitter-receiver pairs having infinite sensitivity photoelectric gradations.
The aforementioned and other problems are solved by the new digital photoelectric sweep method and system described herein.